The prior art includes treatment of plastic films, papers, metal foils, and other materials using a high frequency, high voltage, electrical discharge, or corona. Early equipment employed a spark gap to generate the high frequency required. In the late 1960's, solid state generators were introduced which used SCR's (silicon controlled rectifier). The latest equipment uses IGBT's (insulated gate bipolar transistors) to provide high frequency switching within the power supply, also referred to herein as the "corona treatment generator."
Referring to FIG. 1a, a "conventional" corona treatment station 10a of the prior art is composed of a corona treatment generator 11a, providing high frequency and high voltage, a stationary metal electrode 12a, and an electrode roller 13a covered with a thin layer of dielectric material 14a. The electrode 12a and roller 13a are separated by a small gap 15a, usually about sixty mils, where the corona 16a is formed. Although the elements 12a, 13a are formed as cylinders, only their walls next to the corona gap 15a have been shown in section, it being understood that a full section would show a second thickness of layer 14a, for example. The surface energy of a plastic web of material 17a is raised by direct contact with the corona 16a. This improves the wetting and adhesion of inks or coatings applied in subsequent operations.
The electrode 12a in a conventional system is normally bare aluminum, steel, or stainless, in bar, tube, or flat-sided form. The electrode may be shaped (shoe form) to match the curvature of the covered roller. The electrode may also be one piece, for full-width treatment of the web, or broken into sections (segments) to allow adjustment of the area being treated.
Many types of materials have been used for the dielectric covering 14a on the roller in a conventional system. The oldest types are rubber compounds made from silicone and Hypalon.TM. polymers. Other coverings are made of epoxy and other thermoset resins (solid resin or composites with fibers), glass, and ceramics.
Inorganic coatings, such as glass and ceramic, provide durability and can also provide tolerance to the highest power densities in the corona since they are not combustible. These are more expensive than rubber coverings, for example. Epoxy and ceramic coverings are the most popular dielectrics for conventional systems. The thickness of the polymer coatings is typically 100 to 125 mils, while the thickness of glass and ceramic coatings is much lower, 40 to 80 mils (mainly due to cost). The practical range for all possible dielectric materials is from 20 mils to 500 mils in thickness.
Referring to FIG. 1b, a bare roll form of corona treatment system 10b is shown. In this system 10b, the roller 13b does not have a covering, so the system 10b is referred to as "bare roll". The original dielectric on the upper electrode 12b was quartz but has been replaced with ceramic 18b (aluminum oxide) for improved temperature resistance. The bare roll electrode 12b is usually a tube (or several tubes connected in parallel) of ceramic 18b, filled with metal powder 21b, to avoid the thermal expansion problems of a solid metal electrode. The packed metal powder is connected directly to a high voltage lead from the generator.
The corona 16b of a bare roll electrode system 12b is always full width and cannot be adjusted. The bare roll system 10b has the advantage that the web 17b is in contact with a grounded roller 13b. If a metallic web is being treated, the web 17b will not become energized, due to the grounding. The bare roll system 10b has the disadvantage that it is less efficient than other prior systems. Less input power is delivered from the corona treatment generator 11b to the web 17b. Consequently, the power of a bare roll system 10b must be increased to equal the treatment of other systems, or the line speed must be reduced to reach the desired treatment level.
To overcome or reduce the limitations of the bare roll system, a dielectric cover 19c has been applied to the "bare" electrode roller 13c. The covers 18c, 19c on both the electrode 12c and roller 13c are typically ceramic, although other materials could be used for the roller covering. The added dielectric 19c on the roller 13c shifts the power distribution toward the web 17c, which increases the percentage of the input power actually used to treat the film, however, it does not provide a grounded web.